x86: rust: set rustc-abi=x86-softfloat on rustc>=1.86.0

When using Rust on the x86 architecture, we are currently using the
unstable target.json feature to specify the compilation target. Rustc is
going to change how softfloat is specified in the target.json file on
x86, thus update generate_rust_target.rs to specify softfloat using the
new option.

Note that if you enable this parameter with a compiler that does not
recognize it, then that triggers a warning but it does not break the
build.

Cc: <[email protected]> # Needed in 6.12.y and 6.13.y only (Rust is pinned in older LTSs).
Link: rust-lang/rust#136146
Signed-off-by: Alice Ryhl <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Added 6.13.y too to Cc: stable tag and added reasoning to avoid
  over-backporting. - Miguel ]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: Darksonn

scripts/generate_rust_target.rs

@@ -165,6 +165,18 @@ impl KernelConfig {
         let option = "CONFIG_".to_owned() + option;
         self.0.contains_key(&option)
     }
+
+    /// Is the rustc version at least `major.minor.patch`?
+    fn rustc_version_atleast(&self, major: u32, minor: u32, patch: u32) -> bool {
+        let check_version = 100000 * major + 100 * minor + patch;
+        let actual_version = self
+            .0
+            .get("CONFIG_RUSTC_VERSION")
+            .unwrap()
+            .parse::<u32>()
+            .unwrap();
+        check_version <= actual_version
+    }
 }
 
 fn main() {
@@ -182,6 +194,9 @@ fn main() {
         }
     } else if cfg.has("X86_64") {
         ts.push("arch", "x86_64");
+        if cfg.rustc_version_atleast(1, 86, 0) {
+            ts.push("rustc-abi", "x86-softfloat");
+        }
         ts.push(
             "data-layout",
             "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128",
@@ -215,6 +230,9 @@ fn main() {
             panic!("32-bit x86 only works under UML");
         }
         ts.push("arch", "x86");
+        if cfg.rustc_version_atleast(1, 86, 0) {
+            ts.push("rustc-abi", "x86-softfloat");
+        }
         ts.push(
             "data-layout",
             "e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-i128:128-f64:32:64-f80:32-n8:16:32-S128",